In a worldwide effort to protect the global environment, a growing demand for cleaner exhaust gases and better fuel economy is necessary. A lean-burn engine is an effective technology to improve fuel economy. However, lean-burn engines produce oxygen-rich exhaust gases in which the removal of nitrogen oxides (NOx) is particularly difficult using a conventional 3-way catalyst. In order to overcome this problem, NOx storage-reduction (NSR) catalysts have been developed. The NSR catalyst includes a storage element, typically barium oxide and a reduction-oxidation component, typically Pt. Therefore, during lean operation, NOx species are stored as nitrates, and during rich or stoichiometric operation, the stored NOx is reduced to nitrogen.
A problem associated with a NSR catalyst is the decrease in performance caused by deactivation of the catalyst by sulfur. Sulfur poisoning of the catalyst occurs by a competitive sorption of sulfur oxides (SOx) derived from combustion of sulfur species contained in the fuel. SOx species react with the catalyst in the same way NOx species do, causing the catalyst to deteriorate. Sulfur poisoning occurs in two different ways. One is the oxidation of SO2 by the precious metal to further react with a carrier such as alumina to form aluminum sulfate. The second mechanism is the reaction of SOx with the storage component of the catalyst to form sulfates (BaSO4), which are more stable than the nitrates. Thus, once the storage component contains sulfates, the NSR catalyst can no longer store NOx.
In order to improve the durability of the catalyst, one may suppress sulfur adsorption and promote sulfur desorption. Regeneration of NSR catalysts by desorption of the sulfur species only occurs under reducing atmospheres at temperatures greater than 600° C. Desorption of sulfur species in the form of H2S occurs more readily in a H2 atmosphere at 600° C., than in a propene or carbon monoxide flow, where removal of sulfur occurs at temperatures above 650° C.
Due to the high temperatures required for sulfur removal using conventional heating, there is therefore a need in the art for an improved process for the removal of sulfur from a catalyst.